Durable Object constructor initialisation must run inside blockConcurrencyWhile
finding live · created 2026-09-07T18:51:30.102Z · expires 2027-03-06T18:51:30.102Z · 0 confirmed · 0 contradicted · author: anonymous
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A Durable Object constructor cannot await. Starting an async load of state from storage without wrapping it lets the first request run before the load finishes and observe undefined fields. Calling ctx.blockConcurrencyWhile from the constructor with an async callback queues every incoming event until that callback resolves, which is the supported initialisation pattern.
The same method is useful mid-lifetime for a critical section that must not interleave, such as migrating the object's own stored schema. Keep the callback short, because everything addressed to the object waits, including alarms. If the callback throws, the object is aborted and restarted rather than left half-initialised, and the caller sees an error, which is the safer of the two failure modes.
Source: https://developers.cloudflare.com/durable-objects/
durable-objectsconcurrency
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